The formidable building block for the development of (micro)electronics during the last one-half of the century is the field-effect transistor (FET) based on inorganic electrodes, insulators, and semiconductors. These materials have proven to be reliable, highly efficient, and with performance that increases periodically according to the well-known Moore's law. Rather than competing with conventional silicon technologies, an organic FET (OFET) based on molecular and polymeric materials may find large scale applications in low-performance memory elements as well as integrated optoelectronic devices, such as pixel drive and switching elements in active-matrix organic light-emitting diode (LED) displays, RF-ID tags, smart-ID tags and sensors. These systems have been widely pursued since they offer numerous advantages for easy evaporation/solution processing and good compatibility with a variety of substrates including flexible plastics, and great opportunities for facile structural modifications. This trend is driven by the demand for low-cost, large area, flexible, and lightweight devices and the possibility to process these materials at much lower substrate temperatures as compared to the high substrate temperatures for typical inorganic semiconductors.
The simplest and most common OFET device configuration is that of a thin-film transistor (TFT), in which a thin film of the organic semiconductor is deposited on top of a dielectric with an underlying gate (G) electrode. (See FIG. 1, with dimensions for purpose of illustration only; and other configurations are possible.) In the example shown, charge-injecting drain-source (D-S) electrodes providing the contacts are defined either on top of the organic film (top-configuration) or on the surface of the FET substrate prior to the deposition of the semiconductor (bottom-configuration). The current between S and D electrodes is low when no voltage is applied between G and D electrodes, and the device is in the so called ‘off’ state. When a voltage is applied to the gate, charges can be induced into the semiconductor at the interface with the dielectric layer. As a result, the D-S current increases due to the increased number of charge carriers, providing the ‘on’ state of a transistor. Key parameters in characterizing a FET are the field-effect mobility (μ) which quantifies the average charge carrier drift velocity per unit electric field and the on/off ratio (Ion:Ioff) defined as the D-S current ratio between the ‘on’ and ‘off’ states. For a high performance OFET, the field-effect mobility and on/off ratio should both be as high as possible.
Most of the OFETs operate in p-type accumulation mode, meaning that the semiconductor acts as a hole-transporting material. However, for the full development of the field, and for organic CMOS devices, high-performing electron-transporting (n-type) materials are needed as well. For most practical applications, the mobility of the field-induced charges should be about 0.1-1 cm2/Vs or greater. To achieve high performance, the organic semiconductors should satisfy stringent criteria relating to both the injection and current-carrying phenomena, in particular: (i) the HOMO/LUMO energies of the individual molecules (perturbed by their placement in a crystalline solid) should be at levels where holes/electrons may be added at accessible applied voltages; (ii) the crystal structure of the material should provide sufficient overlap of the frontier orbitals (π stacking and edge-to-face contacts) to allow charge to migrate among neighboring molecules; (iii) the compound should be highly pure since impurities act as charge carrier traps; (iv) the molecules (in particular the conjugated core axes) should be preferentially oriented with their long axes close to the FET substrate normal, as the most efficient charge transport occurs along the direction of intermolecular π-π stacking; and (v) the domains of the crystalline semiconductor should cover uniformly the area between source and drain contacts, hence the film should have a single crystal-like morphology.
Among the organic semiconductors used in OFETs, the class of (oligo, poly)thiophenes are certainly one of the most investigated. The first report on a polyheterocycle-based FET was on polythiophene, and poly(3-hexyl)thiophene and α,ω-dialkyloligothiophenes were the first high-mobility polymer and small molecules, respectively. Over the years, chemical modification(s) of the thiophene core, variations in ring-to-ring connectivity and substitution pattern have resulted in the production and testing of a considerably large number of thiophene-based materials. However, with the exception of very few α,ω-di(cyanomethanide-, perfluorohexyl, and perfluorophenyl)-substituted nTs, all of these materials are p-type semiconductors.
The synthesis of a large number of fluorocarbon-functionalized oligothiophenes was recently described and compared the molecular/solid-state properties with the corresponding alkyl-substituted and the parent unsubstituted oligothiophenes. All fluorocarbon-substituted oligothiophenes considered had large chemical/thermal stabilities, exhibit similar packing characteristics, strong π-π intermolecular interactions, and comparable LUMO energies across conjugation length. Furthermore, fluoroalkyl functionalization of the nT core significantly alters the electronic, film growth, and semiconducting properties of the resulting films, and that a TFT device with these system as active layer operates in the n-type accumulation mode, indicating facile electron injection into the semiconducting material. In addition, film growth morphologies were shown to strongly depend on growth temperature and substrate functionalization. The field effect mobilities measured in the saturated regime (Vd>Vg) approach ≈0.3 cm2/Vs, the highest reported so far for organic n-type semiconductors. See, U.S. Pat. No. 6,585,914 incorporated herein by reference in its entirety. However, such fluorocarbon substituents limit subsequent structural modification and, in certain environments, present concerns regarding chemical stability.